The subject matter of the present invention relates generally to multiple layer optical data records and associated playback apparatus, and in particular to such records which include a plurality of data information tracks in different data layers provided on at least one side of the record substrate or base member, and to such playback apparatus which includes selection means for selectively reading out one of such data tracks with a scanning light beam to produce an electrical readout signal corresponding to the data on the scanned track. The multi-layered records and playback apparatus of the present invention are extremely useful for high density information storage. For example, one application of the present invention is the recording and playing back of audio and video signals used in television programs.
Previously it has been suggested in U.S. Pat. No. 3,430,966 of D. P. Gregg, granted Mar. 4, 1969, and U.S. Pat. No. 3,518,442 of K. O. Johnson, granted June 30, 1970, to provide an optical data record in which data is recorded by notches in two tracks on opposite sides of a light transmissive substrate member. However, such prior art records and their associated playback apparatus have several disadvantages since the records only have one data layer per side and are read out by transmitting a light beam through the record including a light diffusing layer provided between the two data layers. As a result, these records cannot store as much data as the record of the present invention and are more expensive and difficult to manufacture. Also, the playback apparatus requires separate focusing optical systems for the light detector and for the light source because they are on opposite sides of the record. Thus, the playback apparatus is not as compact as the embodiment of the present invention employing reflective records because in the latter the detector is positioned on the same side of the record as the light source.
It has been proposed by K. Compaan and P. Kramer in Philips Technical Review, Volume 33, pages 178 to 180, 1973 No. 7, to employ light reflecting optical data records so that the detector and light source may be positioned on the same side of the record and may use common optical elements. However, this record has the disadvantage that optical data is recorded as pits or notches in a single layer which is formed in a conventional manner by pressing the records from a master. As a result, only about thirty minutes of television programs can be recorded on a large record of 30 centimeters in diameter, the size of a conventional long playing phonograph record. The entire surface of the record, including the data pits, is coated with metal for reflecting the light beam so that several data layers could not be recorded on the same side of the record in the manner of the present invention. The data pits, unlike the background surface, reflect the light beam away from the detector and are detected by the absence of light at the detector so that there is a lower signal to noise ratio in the electrical readout signal than that of the present invention.
Also of interest as a background on optical data records and playback systems for fixed records in U.S. Pat. No. 3,501,586, granted Mar. 17, 1970, to J. T. Russell. However, this patent does not disclose multi-layered data records in the manner of the present invention.